1. Field of the Invention
The present invention is directed generally to a semiconductor memory device and, more particularly, to an equilibrate circuit for dynamic plate sensing semiconductor memories.
2. Description of the Background
In a conventional dynamic random access memory (DRAM) device each memory cell, or memory bit, consists of one transistor and one capacitor. A terminal of the transistor is connected to a digitline, or bitline, of the memory device. Another terminal of the transistor is connected to a terminal of the capacitor and the gate terminal of the transistor is connected to a wordline of the memory device. The transistor thus acts as a gate between the bitline and the capacitor.
The second terminal of the capacitor is connected to a voltage rail which carries a voltage, such as VCC/2. Thus, when the wordline for a particular cell is active, the gate transistor is in a conducting state and the capacitor is connected to the bitline. The capacitor stores a charge that, depending on whether the polarity of the voltage across the capacitor is positive or negative, represents either a logic high or a logic low values
Memory devices are typically constructed with complementary bitlines of equal capacitance. Sense amplifiers are connected between the bitlines and operate to sense the differential voltage across the bitlines. Before a memory cell is selected for access, the complementary bitlines must be equilibrated to minimize the cell access time. Equilibration circuits typically short the complementary bitlines together, resulting in an equilibrate voltage equal to the voltage midpoint between the two equal capacitance and logically opposite bitlines.
In a dynamic plate sensing memory device, each memory cell typically consists of a transistor and a capacitor. The transistor is connected to a bitline and the terminal of a capacitor. The other terminal of the capacitor is connected to a cell plateline. A wordline is connected to the gate terminal of the transistor. Thus, the capacitor is connected between the bitline and the plateline when the wordline is active.
Certain types of dynamic plate sensing memory devices, such as 6F.sup.2 dynamic plate sensing memory devices, do not have complementary bitline architectures. Thus, conventional techniques of equilibrating bitlines, such as that shown in Asakura, et al., "Cell-Plate Line Connecting Complementary Bit-Line (C3) Architecture for Battery-Operating DRAM's", IEEE Journal of Solid-State Circuits, vol. 27, no. 4, April 1992, pp. 597-602, would be ineffective because such conventional techniques require complementary bitlines that can be shorted together to obtain the midpoint of the equal and opposite capacitances of the bitlines.
Thus, the need exists for an equilibrate circuit that is capable of equilibrating the bitlines and the platelines, which have unequal capacitances, of a dynamic plate sensing memory device which does not have a complementary bitline architecture.